The plan was to create the rest of the helmet because the mask only hours up to my ears.
The problem is, though, is that I'm not even sure what the mask is made from and I don't want the rest of the helmet to look and feel too different from the mask. I also don't know what exactly to make the helmet.
Maybe this video will help you guys to figure out the material:

I'd personally stay away from Model Magic. It shrinks a ton and as a result cracks easily. Not to mention it takes 3 or 4 days to truly cure.

When I'm building a helmet from an existing structure I usually adhere the two structures with a suitable glue, then blend the seam line with bondo auto body filler. My Zhou Tai helmet started out as a construction helmet, but with enough bondo you can hide the seams.

This is the same in that your existing structure is on the face, and you have to now make the dome. Here is a link to my build log. Hope it helps.

Fiberglass is what gives it structure. Bondo, while stiff, is also incredibly brittle. If you would like the helmet itself to have any kind of survivability outside of 1 con I would highly recommend glassing the helmet.